The purpose of these studies is to characterize (at the molecular and immunochemical level) the type-D retroviruses recently isolated from nonhuman primate and human cells, to define their distribution in primate tissue, and their relationship, if any, to mammary carcinoma. Major accomplishments during the past year include: (a) delineation of the distribution and diversity of type-D retroviral genes in primates and characterization of their mode of transmission; (b) development of novel group and interspecies radioimmunoassays for the detection and classification of type-D retroviruses of several primate species; (c) the detection of type-D retroviral gene products in primate tissues; (d) immunologic characterization of unique type-D retroviruses isolated from human cells.